'River Capture'

The diversion (capture) of the headwaters of one river into a neighbouring river. River capture occurs when a stream is carrying out rapid headward erosion (backwards erosion at its source). Eventually the stream will cut into the course of a neighbouring river, causing the headwaters of that river to be diverted, or ‘captured’.1 

The additional water flowing down the capturing stream may accelerate erosion and encourage the development of a canyon (gorge).The now-dry valley of the original stream is known as a wind gap.2

Sources:
1. http://www.talktalk.co.uk/reference/encyclopaedia/hutchinson/m0035439.html
2. http://en.wikipedia.org/wiki/Stream_capture